Scrap automotive and truck tires can be recycled into chipped tires (large wire-free shredded chunks) or crumb rubber (fine wire free granular particles). Scrap tires are generally processed into crumb rubber either by the use of cryogenic reduction processes or by mechanical grinding. Cryogenic reduction processes consist of freezing the shredded rubber at an extremely low temperature (far below the glass transition temperature of the rubber), then shattering the frozen rubber into small particles. The cryogenic reduction process is clean and fast, and produces a crumb rubber of a fine mesh size, but is more costly than mechanical grinding. Mechanical grinding consists of mechanically breaking down the rubber into small particles using grinding apparatus, called cracker mills. Cracker mills are well established and can produce crumb rubber of varying particle size, grades and quality at relatively low cost. In the mechanical grinding process, material may be passed through a cracker mill in order to achieve the desired particle size.
In the recycling process, whether using cryogenic or mechanical processes, metal wires and non-metallic fibers (fluff), dust, glass and other debris are removed as the tires are shredded into chipped tires and ground into crumb rubber. Steel fibers are removed using a magnetic separator, sieve shakers, centrifuges, and other mechanisms. Fluff, dust and other debris are separated from the ground crumb rubber by air classifiers or other separation equipment.
The mechanical process of shredding tires into chipped tires and grinding chipped tires into crumb rubber generates considerable heat. Often the temperature of the crumb rubber coming out of a cracker mill reaches the vulcanization temperature of the rubber, where the rubber melts rather defeating the grinding process. Consequently, the crumb rubber coming out of cracker mills must cool before it can be further ground in additional passes through a cracker mill. Air classifiers, and other separators used in the recycling process provide some cooling, but generally are insufficient to cool the crumb rubber so that it can be immediately ground again.